Many grating spectrometers use a folded optical configuration similar to that shown in FIG. 1. The figure shows several aspects of the device that are pertinent to the following background of the art discussion.
The source (not shown) is placed behind the entrance slit (1), and has a divergence which would lead to overfilling of the entrance mirror, M1 (3). Any light that misses M1 (3) will not form a dispersed image at the detector plane (6), but will instead scatter inside the spectrometer, leading to an increase in background signal.
To avoid this stray light problem, an aperture (2) is used after the slit (1) to limit the acceptance cone of the spectrometer to only light that will strike the input minor (3). The aperture (2) size is usually expressed as an f-number by comparing the size of the beam at M1 (3) to the focal length of M1 (3).
M1 (3) is used as an off-axis collimating element. This introduces a variety of aberrations into the collimated beam, primarily astigmatism and coma. The result is that the nominally collimated beam actually contains a distribution of angles.
The diffraction angle from the grating (5) depends non-linearly on the angle of incidence, so the angular distribution present in the incident beam is broadened in the diffracted beam. Furthermore, since the diffraction angle is also a function of wavelength, the output distributions differ for the various wavelength components of the beam.
M2 (4) is also used as an off-axis element, and therefore contributes its own aberrations into the image formed at the detector (6). M2 (4) must be larger than M1 (3) in order to avoid vignetting the dispersed light diffracted from the grating (5). Since different portions of M2 (4) are used by different wavelength components, the aberration contributions are different as well.
These issues are all well-known, and are traditionally addressed by designing the optical system to minimize both the fold angles and the size of the input aperture (2). By holding both parameters to the smallest possible values the aberrations are minimized.